4 Acids and bases
Photolysis processes: Bronsted-Lowry’s conceptions of acids and bases
- Acid: proton (H+) donor tobase
- Base: Proton acceptor (H+)
- Limitations: only protic acids are classified (contains H+)
Thestrengthofanacidandabaseismeasuredwiththeaciddissociation equilibrium constant (Ka)
The larger pKa, the weaker the acid. The larger KA, the stronger the acid.
For bases we use the Base dissociation constant (Kb)
Conjugate acid-base pair
- Conjugate base (CB) = the base formed when an acid loses a proton (Acid - H+)
- Conjugate acid (CA) = an acid formed when a base accepts a proton (base + H+)
- HCL + H20 ⇄ Cl- + H30+
acid base CB CA
Pairs: (HCl/Cl-) (H3O+/H2O)
Strength of acids and bases
- Strong acidscompletely dissociate —>
Weak acids partially dissociate ⇄
Strong bases totally ionise forming OH- in aq solution —>
Weak bases protonate only to a small degree in solution ⇄
CH3COOH + H2O ⇄ CH3COOH- + H3O+
Ka = [CH3COOH-] [H3O+] / [CH3COOH]
products are equal therefore: Ka = [H+]2 / [CH3COOH]
pKa = -log (Ka)
The larger the value of Ka the stronger the acid. Thus, the smaller the value of pKa, the stronger the acid. (A↑, Ka↑, pKa↓)